Hi, is the function module test tool in SE37 the problem ? In the user interface the length seems to be limited to 200 characters.
But in a real ABAP there is no length limitation.
If you still want to call the function from SE37 test tool then you can you the debugger. In the debugger a STRING variable data can be replaced with unlimited length.
For this function module the BASE 64 data is copied in to STRING variable B64COPY en you can change the content in the debugger.
Regards Jack